Conventionally, in order to prevent scratches or light reflection on screens of image display devices, touch panels, or other members, a hard coat film is sometimes provided on such a member.
The hard coat film includes a transparent film and a hard coat layer provided on the transparent film. The hard coat layer is usually formed of a thin coating film obtained by forming a film of about 2 μm to 10 μm from a hard coat layer-forming material containing an ionizing radiation-curable resin such as a heat-curable resin or a UV-curable resin.
In the former hard coat film, a refractive index difference exists between a transparent film and a hard coat layer. Therefore, there is a problem in that the presence of slight irregularities in the surface of the hard coat layer leads to the generation of interference fringes. In addition, “interference fringes” means a striped appearance caused by the reflection of light from a three-wavelength source, such as a fluorescent lamp, on the surface of the hard coat layer, etc. Such interference fringes cause a decrease in the visibility of the surface of the hard coat film.
In a known hard coat film, in order to prevent the generation of interference fringes, the surface of the transparent film is swollen by a hard coat layer-forming material to eliminate an interface between the hard coat layer and the transparent film, whereby the refractive index near the interface continuously varies (Patent Documents 1 and 2).
In another known hard coat film, in order to prevent the generation of interference fringes, an optical adjustment layer is provided between the transparent film and the hard coat layer to eliminate the interface between the transparent film and the hard coat layer (Patent Document 3).
However, the former hard coat films mentioned above do not sufficiently prevent the generation of interference fringes. Further, the hard coat film of Patent Document 3 requires an additional step of providing an optical adjustment layer during its production.
Meanwhile, in a hard coat film, it is necessary that the hard coat layer has high hardness for the purpose of preventing scratches, and, in addition, it is also necessary that the hard coat layer and the transparent film are sufficiently attached to each other.
[Patent Document 1]
Japanese Unexamined Patent Publication No. 2004-263082
[Patent Document 2]
Japanese Unexamined Patent Publication No. 2003-131007
[Patent Document 3]
PCT International Publication No. WO 2006/098363
It is an object of the present invention to provide a hard coat film in which the generation of interference fringes can be further suppressed and which has high hardness and excellent adhesion.
It is another object of the present invention to provide a polarizing plate and an image display device using the hard coat film.
It is another object of the present invention to provide a method for producing a hard coat film in which the generation of interference fringes is suppressed.
It is considered that in the hard coat films of Patent Documents 1 and 2 mentioned above, interference fringes due to the interface that separates a transparent film from a hard coat layer can be suppressed.
However, in such a hard coat film, as a whole, interference fringes are still present.
The present inventors have presumed that this is attributable to the presence of a new additional interface in addition to the interface that separates a transparent film from a hard coat layer. Specifically, the hard coat films of Patent Documents 1 and 2 are obtained by applying a hard coat layer-forming material, which contains a curable resin and a solvent, onto a transparent film, and then curing the coating film. The solvent swells the surface of the transparent film, and therefore, a transparent film-forming component elutes into the coating film. Then, it is presumed that the transparent film-forming component that has diffused and penetrated into the coating film causes the formation of the new additional interface mentioned above. Because of the presence of the new additional interface, practically, the former hard coat film is not capable of sufficiently suppressing the generation of interference fringes. Particularly in the case where the hard coat layer has a small thickness, interference fringes are likely to occur due to the additional interface. Such interference fringes are prominently visible in the appearance particularly when an antireflection layer is provided on the surface of the hard coat layer.
Based on this knowledge, inventors of the present invention solved the problems with the following methods.
A hard coat film according to a first aspect of the present invention includes a transparent film and a hard coat layer provided on the transparent film, wherein an interface that can be detected by a reflection spectrum analysis exists between the transparent film and the hard coat layer and a refractive index continuously varies in the thickness direction in a region from the interface to a thickness-wise middle part of the hard coat layer without forming an interface that can be detected by the reflection spectrum analysis.
A hard coat film according to a second aspect of the present invention includes a transparent film and a hard coat layer provided on the transparent film, wherein an interface that can be detected by a reflection spectrum analysis exists between the transparent film and the hard coat layer and a transparent film-forming component and a hard coat layer-forming component are present in a region from the interface to a thickness-wise middle part of the hard coat layer, and the transparent film-forming component in the region decreases toward a surface of the hard coat layer without forming an interface that can be detected by the reflection spectrum analysis.
In a preferable hard coat film of the present invention, the hard coat layer has a thickness of 3 μm to 15 μm.
In a preferable hard coat film of the present invention, an antireflection layer is further provided on a surface of the hard coat layer.
In a preferable hard coat film of the present invention, a transparent film-forming component and a hard coat layer-forming component are present in a region from the interface to a thickness-wise middle part of the transparent film, and the transparent film-forming component in the region decreases toward a surface of the hard coat layer.
According to another aspect of the present invention, a polarizing plate is provided.
The polarizing plate includes the hard coat film mentioned above.
According to another aspect of the present invention, an image display device is provided.
The image display device includes the hard coat film mentioned above.
According to another aspect of the present invention, a method for producing a hard coat film is provided.
The method for producing a hard coat film of the present invention includes a coating film-forming step of applying a hard coat layer-forming material onto a transparent film to form a coating film having a predetermined thickness, the hard coat layer-forming material including a curable compound containing a low molecular-weight component with a molecular weight of 800 or less and a solvent containing a good solvent for the transparent film, and a curing step of curing the coating film to form a hard coat layer on the transparent film, wherein the low molecular-weight component and good solvent contained in the hard coat layer-forming material and the thickness of the coating film satisfy the relationships of the following formulae 1 and 2, Formula 1: Y≧−4.274 ln (X)+11.311 and Formula 2: Y≦−4.949 ln (X)+15.474. In the formula, the “Y” represents b×t, the “X” represents a×t, the “a” represents the content ratio of the low molecular-weight component based on the total amount of the curable compound taken as 1, the “b” represents the content ratio of the good solvent based on the total amount of the hard coat layer-forming material taken as 1, and the “t” represents the thickness μm of the coating film.
In a preferable method for producing a hard coat film of the present invention, the “a” is 0.3 or more and 1 or less and the “b” is 0.05 or more and 0.5 or less.
In a preferable method for producing a hard coat film of the present invention, the low molecular-weight component is a UV-curable resin monomer or oligomer.
The hard coat film of the present invention is resistant to scratches and also is capable of further suppressing the generation of interference fringes. Further, the hard coat film is also excellent in terms of adhesion between the transparent film and the hard coat layer and thus has excellent durability.
The polarizing plate and an image display device including the hard coat film have excellent scratch resistance, durability, and visibility.
The production method of the present invention makes it possible to easily produce a hard coat film in which the generation of interference fringes is suppressed.